Gerber

After a 1NT or 2NT bid, a jump to 4C is Gerber and asks partner to
respond with a bid that indicates the number of aces he holds.
4D indicates zero or four aces, 4H one, 4S two, and 4NT three. The
4D response is not ambiguous because if a partnership is even
considering the possibility of slam in no-trump, they cannot possibly
lack all the aces (16 of the 40 high card points).

After the response, 4C bidder may bid 5C, which guarantees their
side holds all four aces, and asks partner to indicate the number
of kings he holds in the same manner as for the aces above.

Some partnerships play Gerber in any auction sequence in which a
notrump contract is implied, while others play it only directly after
a notrump bid (and then, after 3NT it is not Gerber because 4C is the
lowest possible bid above 3NT). Use Blackwood in other situations
to ask for aces.

Gerbers are different from the competing Autoplot format in that they are comprised of two files. One of the files (the actual "Gerber file") contains the 2Dcoordinates of features on a single z-plane. The other file, known as the "environment" or "d-code list," defines the shape and size of those features. The default file extension for a Gerber is .gbr, though many programs and design companies use other extensions.